This invention relates generally to carbon nanotubes. More particularly, carbon nanotubes including single walled carbon nanotubes adaptable, for example, for atomic probe microscopy.
Atomic force microscopy has been a powerful tool for a wide range of fundamental research and technological applications. At the heart of the AFM lies the probe tip, whose size and shape dictate the lateral resolution and fidelity of AFM images.
Carbon nanotubes are recently discovered, hollow graphite tubules. When isolated, individual nanotubes are useful for making microscopie electrical, mechanical, or electromechanical devices. Carbon nanotubes are useful in a variety of applications, including chemical, mechanical and electrical applications.
One particular application for which carbon nanotubes are useful is atomic force microscopy (AFM). Atomic force microscopy has been a powerful tool for a wide range of fundamental research and technological applications. At the heart of the AFM lies the probe tip, whose size and shape are related to the lateral resolution and fidelity of AFM images. Atomic force microscopes (AFMs) sometimes employ nanotubes as the scanning tip because nanotubes are resilient and have an atomically sharp tip. Carbon nanotube tips present ideal characteristics for enhancing the capabilities of AFM in imaging, manipulation and nanofabrication due to their sharpness, high aspect ratios, high mechanical stiffness and resilience, and tunable chemical characteristics. Nanotube tips exhibit convincing advantages including longer durability, the ability to probe deep structures, and to achieve high lateral resolution in imaging and lithographic applications. Individual SWNT tips, once realized, can be as small as 7 angstroms in diameter. These tips should be ideal for real-space imaging of a wide range of systems with  less than 1 nm lateral resolution and for nanofabrication of new generations of molecular devices.
Obtaining individual, high quality, single-walled nanotubes has proven to be a difficult task, however. Existing methods for the production of nanotubes, including arc-discharge and laser ablation techniques, yield bulk materials with tangled nanotubes. The nanotubes in the bulk materials are mostly in bundled forms. These tangled nanotubes are extremely difficult to purify, isolate, manipulate, and use as discrete elements for making functional devices.
A conventional arc discharge method for producing carbon nanotubes is disclosed in U.S. Pat. 5,482,601 issued to Oshima et al. on Jan. 9, 1996. Oshima describes a discharge between a carbon anode and a cathode surface that produces carbonaceous deposits containing carbon nanotubes. U.S, Pat. 5,500,200 issued to Mandeville et al. on Mar. 19, 1996 discloses a catalytic method for the bulk production of multi-walled tubes. According to the method, a catalyst is prepared using particles of fumed alumina with an average particle size of about 100 xc3x85. Iron acetylacetonate is deposited on the alumina particles, and the resultant catalyst particles are heated in a hydrogen/ethylene atmosphere.
Although the methods described by Oshima and Mandeville are effective for producing bulk amounts of carbon tubes or carbon fibrils, the resulting bulk materials generally comprise tangled and kinked tubes. These bulk materials are useful as additives for improving the material properties of polymer or metal composites. However, it is nearly impossible to isolate one individual tube from the tangled material, manipulate the tube, and construct a functional device using that one tube. Therefore, these bulk materials are nearly useless for making functional nanotube-tipped AFM devices. Furthermore, many of the tubes have molecular-level structural defects, which results in weaker tubes with poor electrical characteristics.
Recently, Hafner et al. described the use of chemical vapor deposition methods to directly grow multi-walled nanotubes (MMTs) and single-walled nanotube (SWNT) bundles on silicon to synthesize nanotube AFM tips. U.S. patent application Ser. No. 09/133,948 to Dai et al. Describes a catalytic CVD technique that uses catalyst islands to grow individual nanotubes for AFM applications. The catalyst island includes a catalyst particle that is capable of growing carbon nanotubes when exposed to a hydrocarbon gas at elevated temperatures. A carbon nanotube extends from the catalyst particle. In this way nanotube, AFM tips have been obtained by attaching MWNTs and SWNT bundles to the sides of silicon pyramidal tips. Unfortunately, it is difficult to deposit the catalyst particles onto a support structure, such as a silicon pyramid.
In addition, SWNTs extending from silicon pyramid tips typically range 1-20 microns in length beyond the pyramid tip. The nanotubes are usually shortened to xcx9c30-100 nm in a reactive discharge to obtain rigid AFM probe tips needed for imaging. The shortening mechanism involves etching the nanotube by reactive species generated in the discharge process. Under ambient conditions each shortening step of such a discharge process typically removes a xe2x89xa7100 nm long nanotube segment. Often, the entire SWNT beyond the pyramid tip is inadvertently removed at the final shortening step attempting to reach a desired length of 30-100 nm. Consequently the shortening process is difficult to control.
It would be an advance in the art of carbon nanotubes and atomic force microscopy to provide mass production of individual nanotubes thereby making a carbon nanotube useful for example, in a nanotube-tipped AFM device that is simpler to manufacture.
According to one example embodiment of the present invention, carbon nanotubes including oriented single-walled carbon nanotubes are grown on silicon tips. In one particular implementation, AFM tips ate formed with individual SWNTs of radii as small as xcx9c1 nm. In another implementation, arrays of nanotube tips are formed for use in atomic probe microscopy. In still another implementation, nanostructures having a feature size below 10 nm are fabricated.
In another example embodiment of the present invention, carbon nanotubes are fabricated using a liquid phase precursor. According to a first embodiment, a support structure is formed and a portion thereof is coated with the liquid phase precursor material. One or more nanotubes are then grown from the support structure by exposing the coated support structure to a carbon containing gas; and heating the structure in a heating environment. The liquid-phase precursor material typically comprises a metal-containing salt and a long-chain molecular compound dissolved in a solvent. Exposing the heating environment to a catalyst may enhance growth of nanotubes.
A second embodiment fabricates carbon nanotube probe tips in large scale arrays. This method forms a substrate having an array of support structures. A stamp is then coated with a liquid phase precursor material. The stamp then contacts the support structure array to transfer the precursor material to the support structures. The array is then exposed to a carbon containing gas and heated in a heating environment to form carbon nanotubes on the support structures. In one implementation, the liquid phase catalyst does not coat a portion of the substrate between the support structures.
According to a third embodiment, the support structure is in the form of a pyramid or cone shaped tip on top of a tower. According to a fourth embodiment of the present invention, carbon nanotubes are shortened in a controllable fashion using an inert discharge.